This invention is generally directed to an icebreaker vessel. More specifically, the invention is directed to a separately-powered icebreaker attachment for a ship.
A conventional icebreaker vessel is especially designed for icebreaking. Since the operating season of an icebreaker is normally only a few months in each year, during the greater portion of each year the money invested in an icebreaker is non-performing. Also, the desirability of navigation in cold climates is offset by the tremendous capital cost of building large icebreaker vessels for use (for example) on the Great Lakes. Therefore, there is a need for a a vessel that can be used both as an icebreaker and also, during the "off-season," for off-shore supply, diving support, towing, research, entertainment, or other purposes.
Contemporary ice breaking is performed by icebreakers in two principal modes: (1) a continuous mode, in which the ship is driven forward through the ice at varying speeds (restrained only by the ice resistance), but during which forward movement is typically never totally impeded; and (2) a ramming mode, in which the icebreaker encounters ridge ice of such a thickness that the forward motion cannot be maintained continuously and the ship comes to a stop after having crushed the ice under her forefoot. In the ramming mode, the ship is then backed away from the ice an appropriate distance and then again moved forward against the ice. An icebreaker must be designed for safe and efficient operation in each of these ice breaking modes.
One principal reason for using an icebreaker is to provide safe travel for vessels that do not possess ice breaking capability. Thus, the icebreaker should be able clear a channel equal to the width of the ship's beam, without leaving large chunks of ice strewn throughout its wake. Such large ice chunks can be of considerable size and weight, and can continue to provide a hazard to the icebreaker hull, propellers and rudders, while smaller ice pieces can move under the hull and clog underwater hull openings such as sea chests and thruster ports. In addition to preventing these hazards, the formation of a clear path allows escorted vessels to travel more safely, and also reduces the amount of brash ice (or residual ice that can refreeze) so that other vessels can use the path for a longer period of time. Icebreakers are also used for other related duties such as channel widening, removing floating ice chunks, providing turnout points and turning basins, and harbor clearing. Also, icebreakers are required to free ships which are already locked in ice. This requires good maneuvering characteristics to work in close proximity with another ship. It would be preferable to provide an icebreaker that could efficiently perform each of these duties, as well.
To perform such duties as channel clearing and widening, icebreakers require a relatively wide hull. This results in poor performance during open water operations, since such hull designs do not efficiently dampen the rolling motion in open sea, and do not efficiently cut through the open water at relatively high travelling speeds.
Conventional icebreaker ships have a V-shaped bow with a wedge extending from the bottom of the stem line below the design waterline of the ship, towards the sides, until a maximum width is reached. Upon breaking of the ice by the bow, the cusps of ice move downwardly into the water along the sides of the bow until the wedge is contacted. It is then the design intention that the cusps of ice are tripped and moved away from the ship's sides and under the unbroken ice, thus protecting the propellors and leaving a clear channel behind the icebreaker. In practice, these design objectives are not always achieved.
A spoon-shaped icebreaker bow has also been used, as disclosed for example in U.S. Pat. No. 4,702,187. Relatively low resistance to breaking level ice is achieved with a spoon bow as compared to a wedge bow. The spoon bow breaks ice by riding up on the ice field until there is enough downward force to cause ice failure in the flexure mode. A spoon bow also has very good ice ridge penetration characteristics because it breaks the ice with a downward force instead of attempting to wedge the ridge apart. The energy dissipated during each ramming sequence is reduced, allowing the spoon bow to penetrate further into the ridge during each ramming cycle and reducing the overall number of backing and ramming cycles. Compared to traditional bow forms, which attempt to wedge the ice to either side, the spoon bow is much more efficient as it transfers more of the propulsion energy directly into icebreaking-related forces. Again, as with "wedge" bows it is important for spoon-shaped bows to clear the broken ice field by pushing the ice chunks underneath the ice field on either side of the icebreaker. For this purpose, a wedge-shaped ice knife has been used with a spoon-bow form.
Various icebreaker designs have been tried in an attempt to achieve some of the above-mentioned objects. For example, U.S. Pat. No. 5,218,917 discloses an icebreaker with differently-shaped fore and aft bows, so that the vessel can be turned around to move in the aft direction during non-icebreaking conditions. As another example, U.S. Pat. No. 4,436,046 mentions icebreakers that utilize explosive devices for breaking up the ice.
Accordingly, it is an object of the present invention to provide an icebreaker attachment that can be used with a conventional non-icebreaker "parent" vessel, and that can be removed to allow the parent vessel to be economically used for other activities, including efficient travel over open water.
It is a further object of the present invention to provide an icebreaker attachment that employs an efficient, spoon-shaped bow.
It is still another object to provide an ice breaking design that leaves a relatively clear channel in the wake of the ship employing the icebreaker attachment.
It is yet another object to provide a removable and attachable icebreaker that can safely and efficiently perform the various duties of an icebreaker.
Another object is to provide a parent vessel with an icebreaker attachment that itself can function as a separate floating vessel, facilitating attachment and detachment of the icebreaker to a parent vessel.
Still another object of the present invention is to power the parent ship/icebreaker combination so that the combined vessel can move efficiently through the ice.